diff --git a/apps/pwa/src/main.tsx b/apps/pwa/src/main.tsx
index dd5c0ab..4f22a91 100644
--- a/apps/pwa/src/main.tsx
+++ b/apps/pwa/src/main.tsx
@@ -28,7 +28,14 @@ import { useCalendarStore } from './store/calendarStore.js'
  * Confirmed via Context7 /tanstack/query docs for v5.101.0.
  */
 function onGlobalError(error: unknown): void {
-  if (error instanceof SessionExpiredError) {
+  // WR-05: instanceof can fail if client.ts is ever loaded through two module graphs
+  // (Vite SSR, duplicated chunk, or the tests' repeated `await import('./client.js')`),
+  // leaving the session-expiry interstitial unarmed. SessionExpiredError carries a fixed
+  // `name` for identity stability, so also check that defensively.
+  if (
+    error instanceof SessionExpiredError ||
+    (error as { name?: string } | null)?.name === 'SessionExpiredError'
+  ) {
     useCalendarStore.getState().setSessionExpired(true)
   }
 }